Conventionally, pneumatic tires with an object of securing straight-line stability are known. For example, the pneumatic tire of Japanese Unexamined Patent Application Publication No. 2002-29216A comprises land portions defined by grooves that intersect a tread width direction cross section in the tread portion. When viewed in a tread width direction cross section, the ground contact surface of the land portions is curved so as to protrude to the outer side in the radial direction, and the apex of the ground contact surface closest to the profile line of the tread surface across the entire tread width is offset from the lateral center of the land portion toward one side edge of the land portion by from 0.1 to 0.4 times the width of the land portion.
Also, Japanese Unexamined Patent Application Publication No. 2004-122904A, for example, describes a pneumatic tire comprising a center land portion extending along the tire equator, an outer land portion extending in the shoulder portion, an intermediate land portion located between the center land portion and the outer land portion; the land portions on a tread surface being defined by circumferential grooves. In such a pneumatic tire when in a regular state of being assembled on a regular rim, inflated to a regular internal pressure, and having no load applied, when viewed in the tire meridian cross section including the tire axis, the radius of curvature R1 of the outer surface of the center land portion is greater than the radius of curvature R2 of the outer surface of the intermediate land portion, and the center of each radius of curvature R1, R2 lies is at the same position.
In addition, Japanese Unexamined Patent Application Publication No. 2011-230699A, for example, describes a pneumatic tire comprising at least one carcass and belt layer, a belt cover layer formed by spirally winding a reinforcing cord made of an organic fiber cord in the tire circumferential direction on the outer side of the belt layer in the tire radial direction, and a tread portion comprising a tread surface different in the groove surface area ratio on either side of the tire equatorial plane. The tread surface of such a pneumatic tire has a tread profile in which, when viewed in the tire median cross section, the outermost position in the tire radial direction is located on the side of the tire equatorial plane with the smaller groove surface area ratio, and shoulder drop amounts in the tire radial direction from the outermost position in the tire radial direction at both ends in the tire width direction are equal.
The pneumatic tire described in Japanese Unexamined Patent Application Publication No. 2011-230699A is provided with a tread portion that comprises a tread surface in which the groove surface area ratio is different on either side of the tire equatorial plane. Water drainage performance is obtained from the side with the greater groove surface area ratio (that is, the side with more grooves), and steering stability on dry road surfaces is obtained from the side with the smaller groove surface area ratio (that is, the side with less grooves and thus greater rigidity). As a result, both water drainage performance and steering stability can be obtained. Moreover, the tread surface of the pneumatic tire has a tread profile in which, when viewed in the tire median cross section, the outermost position in the tire radial direction is located on the side of the tire equatorial plane with the smaller groove surface area ratio (that is, the side of the tread portion having greater rigidity). The ground contact pressure at the side with the greater rigidity is increased, uniforming the ground contact pressure across the entire ground contact region. As a result, increases in conicity due to a difference in rigidity of the tread portion on either side of the tire equatorial plane are suppressed without any negative impact on durability at high speeds.
In recent years, in line with improvements in vehicle performance, pneumatic tires that can achieve both steering stability at high speeds and durability at high speeds when a camber is applied have been demanded. To cater to such a demand to ensure steering stability, a pneumatic tire such as that described in Japanese Unexamined Patent Application Publication No. 2002-29216A has been effective. The pneumatic tire of Japanese Unexamined Patent Application Publication No. 2002-29216A has a configuration in which a rib (land portion), formed in the tread portion, has a profile that, when viewed in the tire meridian cross section, protrudes to the outer side in the tire radial direction beyond the profile of the tread surface so that the rib has better contact with the ground. However, when a camber is applied to a vehicle, durability at high speeds tends to decrease. This is because, in the case of a negative camber, ribs located on the inner side of the tire equator plane when the tire is mounted to a vehicle have a longer footprint length than ribs located on the outer side of the tire equatorial plane. Consequently, achieving both steering stability at high speeds and durability at high speeds when a camber is applied has been difficult.
The pneumatic tire described in Japanese Unexamined Patent Application Publication No. 2004-122904A is designed for enhanced durability. However, the pneumatic tire is for light trucks. Also, the pneumatic tire has a configuration in which, due to differences in the radius of curvature, a center land portion protrudes to the outer side in the tire radial direction a great deal more than the intermediate land portion. Consequently, in the case of the pneumatic tire being applied to a vehicle with a camber, durability of a center land portion decreases and steering stability also decreases.
In addition, the pneumatic tire described in Japanese Unexamined Patent Application Publication No. 2011-230699A comprises a tread profile in which the outermost position in the tire radial direction is located on the side of the tire equatorial plane with the smaller groove surface area ratio, namely the side of the tread portion with the greater rigidity. However, the pneumatic tire described in Japanese Unexamined Patent Application Publication No. 2011-230699A specifies the profile of an entire tread surface including the grooves, yet in essence, the portion that comes into contact with the ground is the land portions (ribs) and not the grooves. In other words, the pneumatic tire described in Japanese Unexamined Patent Application Publication No. 2011-230699A lacks specifics regarding the land portions and the grooves. As a result, it is difficult to suppress increases in conicity to a high degree leaving room for enhancement.